The present invention relates to a passive infrared alarm device comprising an infrared radiation detector, an optical device which directs the infrared radiation from one or more than one angle which is to be monitored towards the detector, an analysis device which processes an output signal supplied by the detector and possibly triggers an alarm signal, and a radiation filter which is located in front of the detector and withholds undesired radiation from the detector up to a limit wavelength.
A passive infrared alarm device of this kind is known and described, for example, in British Pat. No. 1,335,410 (German As No. 21 03 909). As a passive motion alarm, it is based on the principle of recording the characteristic radiation of a room or monitored sections of a room and analyzing any changes in the recorded value as significant of the entry of a person into the monitored room for alarm purposes. For example, an analysis device of this type responds to a change in the infrared radiation with the frequency 0.2 to 5 Hz. This selected frequency range allows differentiation between a change in radiation produced by a person entering or leaving the monitored room and a change in radiation caused, for example, by temperature changes in the room or in the environment.
The advantage of a passive alarm device consists mainly in that no active signal from a monitoring device is present and, therefore, does not produce the possibility of discovery.
The radiation of the room or of monitored sections of the room is focused via an optical device onto an element--the infrared radiation detector--which responds to infrared radiation. This can be carried out via a lens system or via appropriately shaped reflectors or via a combination of the two. A radiation filter is arranged prior to the detector. The radiation filter is to prevent false alarms triggered by reflected sunlight or by other light sources such as, for example, incandescent lamps and fluorescent lamps. It transmits radiation with a wavelength from approximately 4.5 .mu.m to 20 .mu.m and screens other radiation from the detector. As window panes are only permeable to beams of up to a maximum of 4 .mu.m wavelength, beams entering through the window panes from outside of the enclosed, monitored room are withheld from the detector. The radiation filter is constructed by providing a detector input window with a germanium layer upon which a thin layer of dielectric material is vapor deposited. This thin layer reflects radiation up to a limit wavelength of approximately 7 .mu.m for the main part. However, a part thereof is absorbed in the dielectric itself and transmitted to the germanium where it is absorbed. Germanium itself absorbs electromagnetic radiation of up to 1.8 .mu.wavelength. As a result of the absorption, the detector input window is heated and can itself trigger an alarm as a result of characteristic radiation. Therefore, an obviously unintended alarm can be triggered by strong light sources such as car headlights and sunshine passing through the window panes of the monitored room.